Methods for manufacturing a MEMS variable capacitor and a MEMS switch using a MEMS actuator draw increasing attention. MEMS elements are expected to be applied to the MEMS variable capacitor and MEMS switch for high frequency technical field such as cellular phones, automobile telephones, etc. for the following reasons. The MEMS variable capacitor has a higher Q value than a semiconductor variable capacitor conventionally used. The MEMS switch has a lower loss when the switch is ON and a higher insulating characteristic when the switch is OFF than a semiconductor switch conventionally used.
JP-A 2007-259669 discloses a piezoelectric actuator without slits in order to reduce warpage of the beam thereof due to a residual strain. The strength of the beam increases by employing a slit-less structure of a lower electrode film of the beam. This makes the warpage of the beam difficult. However, the piezoelectric actuator has a problem that the operating voltage thereof increases in accordance with an increase in the strength of the beam. JP-A 2008-005642 (corresponding to U.S. Patent Application Publication 20080042521A1) discloses a piezoelectric actuator with a folded beam structure. The folded structure allows it to cancel out the warpage of the beam. However, the actuator has another problem that the resistance of the electrode film lengthened owing to the folded structure increases the output signal loss thereof and decreases the Q value thereof.
In order to reduce the loss of variable capacitors and the passing loss of switches, it is suggested that the resistance of the electrode film is lowered. As specific methods for reducing the resistance of the electrode film of a piezoelectric-driven MEMS element, employing a low resistivity material for the electrode films, shortening the length of the electrode films, widening the width of the electrode films, and thickening the electrode films are suggested on the basis of R=ρ·(L/w·t).
However, Al, Cu, Mo, etc. are currently employed for the electrode films. This gives rise to a small range of choice among the above methods. The length of the electrode films is shortened to increase the operating voltage of the actuator in proportion to the square of the length thereof. The width of the electrode films is widened to make the beam warp easily in a width direction of the electrode films, the beam being hard to bend. There also exists a problem that the operating voltage increases in proportion to the thickness of the electrode film. Lowering the resistances of the electrode films gives rise to a risk of deteriorating the characteristics of variable capacitors or switches. Therefore, it has been difficult to raise Q value of the piezoelectric-driven MEMS element by lowering the resistances of the electrode films.